There is known a rotary valve consisting of a rotor defining a fluid transport passageway and a casing rotatably housing the rotor and configured such that the valve can be switched over between an opened state in which a plurality of ports formed in the case are communicated with each other through the passageway and a closed state in which the ports are blocked.
During rotation of the rotor of the rotary valve, sliding resistance of a port seal is generated relative to the casing. For this reason, a large driving force is needed for rotating the rotor during switchover of the rotary valve. This need invites enlargement of the rotary valve apparatus. Further, for obtaining such large driving force, a large amount of electric power is consumed. So, when such rotary valve is mounted in a vehicle, there is a concern about deterioration of the fuel consumption efficiency.
In the case of a rotary valve disclosed in Patent Document 1, in an attempt to decrease the sliding resistance, the center of curvature radius of the port seal portion is offset so as to allow the rotor to rotate without sliding against the casing, when the rotary valve is switched from the closed state to the opened state.
Further, in the case of a rotary valve disclosed in Patent Document 2, in an attempt to decrease the sliding resistance, a recess is formed in at least either one of an inner peripheral face of the casing and an outer peripheral face of a rotational member of the rotary valve, thereby to decrease the area of the contact between the casing and the rotational member.